Hitherto, a diffusion transfer photographic process utilizing a silver salt such as silver halide is well known in the art. In such a photographic process, a photographic material prepared by coating a film of a polymeric material such as polyethylene terephthalate, cellulose diacetate, cellulose triacetate, cellulose nitrate, polycarbonate, polyvinyl chloride, etc., or a paper or a baryta-coated paper with a dispersion of fine grains of a photosensitive silver salt such as a silver halide in a hydrophilic binder such as gelatin, polyvinyl alcohol, carboxymethyl cellulose, polyvinylpyrrolidone, methyl cellulose, etc., is imagewise exposed as a function of incoming electromagnetic rays and developed by contacting with a developer containing a developing agent. In this case, the light-exposed silver halide in the photosensitive layer is reduced to form non-diffusible silver. When the photosensitive layer is brought into contact with a water-soluble silver complex salt-forming agent simultaneously with or after the above-described development, the unexposed silver halide reacts with the aforesaid water-soluble silver complex salt-forming agent to form a water-soluble silver complex salt. In this case, when an image-receiving element having a layer (image-receiving layer) containing therein a material (i.e., silver depositing nuclei, so-called physical development nuclei or centers), which becomes a catalyst for the reduction reaction of the above-described water-soluble silver complex salt dispersed in a hydrophilic binder is closely brought into contact with the above-described photosensitive layer, the silver complex salt formed in the photosensitive layer diffuses from the photosensitive layer into the image-receiving layer by the existence of a processing liquid, and is reduced to silver by the action of the development nuclei in the image-receiving layer. That is, a silver image is formed in the image-receiving layer as if an image was transferred from the photosensitive layer into the image-receiving layer. From such a technical viewpoint, the above-described photographic process is called a "silver salt diffusion transfer photographic process".
Image-receiving elements for diffusion transfer photographic processes containing a silver depositing agent in an alkaline processing composition-permeable matrix material are known, as described, for example, in U.K. Pat. No. 1,149,921.
In the field of silver salt diffusion transfer photography, the image-receiving element for the photographic process has hitherto been extensively investigated. For example, as the silver depositing nuclei, sparingly water-soluble metal sulfides, metal selenides, heavy metal or noble metal colloids are usually used, and it is preferred that the silver depositing nuclei for the image-receiving element be highly active. For example, U.S. Pat. No. 2,698,237 describes a process of obtaining silver depositing nuclei having a high activity by mixing a water-soluble metal salt and a water-soluble sulfide in a colloidal silica to form a precipitations of water-insoluble metal sulfide. Also, Japanese Patent Publication No. 32754/69 describes an image-receiving element prepared by incorporating a silver depositing nucleus material in an alkali-impermeable polymer material by a vacuum vapor deposition method, dissolving the polymer material in a solvent for the polymer material coating the polymer solution on a support followed by drying, and rendering the surface layer of the polymer layer alkali permeable by a chemical treatment such as hydrolysis, etc.
Also, Japanese Patent Application (OPI) No. 73150/73 (the term "OPI" as used herein refers to a published unexamined Japanese patent application) describes an image-receiving element for the silver halide diffusion transfer process, prepared by hydrolyzing a cellulose ester layer and incorporating silver depositing nuclei in the hydrolyzed layer simultaneously with or after the hydrolysis. However, the image-receiving element prepared in the aforesaid manner has a disadvantage in that the silver image formed in the image-receiving element is liable to discolor or fade during storage of the image-receiving element.
As a method for overcoming the disadvantage, a method has been proposed of coating a water-soluble polymer liquid containing an alkali neutralizing component on the surface of the silver image thus formed, as described in Japanese Patent Publication No. 5392/71, U.S. Pat. No. 3,533,789, and U.K. Pat. No. 1,164,642. However, this method encounters a disadvantage in that it requires a considerable time to completely dry the surface coated with the aqueous polymer solution, and hence the surface is sticky for a long period of time, during which a print cannot be placed on the surface, and a fingerprints or dust may frequently stick to the surface. Furthermore, the coating operation of such an aqueous solution on the surface of silver image is troublesome.
In U.S. Pat. No. 3,607,269, corresponding to Japanese Patent Publication No. 44418/81, an image-receiving material for the silver salt diffusion transfer process is disclosed, which comprises a support having formed thereon (I) a layer of a cellulose ester, a polyvinyl ester, or a polyvinyl acetal, which is hydrolyzable and becomes alkali soluble when it is hydrolyzed, containing a diffusible compound suitable for modifying the photographic properties of transferred silver images, and (II) a reproduced cellulose layer containing silver depositing nuclei, said layer (I) containing no silver depositing nuclei and said layer (II) containing no aforesaid diffusible compound. Also, as the aforesaid diffusible compound, an organic mercapto compound is described.
Further, according to the above-noted U.S. Pat. No. 3,607,269, it is described that the diffusible compound is dispersed in the layer under the image-forming layer prior to the diffusion transfer processing, and a toning agent and a stabilizer are released from the lower layer during diffusion transfer processing, whereby the effect of the diffusible compound during the diffusion transfer process is increased.
Also, it is clearly described in the aforesaid U.S. Pat. No. 3,607,269 that layer (I) and layer (II) are not independent or separate layers, but rather a single continuous layer having partially different properties in the depth direction. When these layers are formed as plural continuous coatings, they are coated using a common solvent, and they are formed into a single layer without forming any interface; it is considered that this feature is an essential characteristic for obtaining the above-described effect. According to the above-described technique, at least a toning agent is present in the layer (I) at the first stage of development, and it diffuses into the layer (II) with the progress of development to act therein as toning agent. As well known to those in the art, a toning agent has an action of changing the color of images by acting at the step of forming developed silver and giving influences on the surface form and other optical properties of the developed silver thus formed; hence, the effect is not obtained if the toning agent does not diffuse into the upper layer from the lower layer.
However, the technique described in U.S. Pat. No. 3,607,269 is very conceptional, and involves many difficult problems in practical use. Hence, it is difficult to use from a practical viewpoint, as is clear to researchers engaging in this art.
That is, first, even when the diffusible compound is incorporated in the layer (I) only in the step of forming the image-receiving material, the mercapto-substituted compound such as 1-phenyl-5-mercaptotetrazole, imidazolidinethion, etc., which is illustrated as the diffusible compound in the patent, is very soluble in the organic solvent which is used for coating, the layer (I) is swelled by the coating solvent for the layer (II) when coating the layer (II) on the layer (I), whereby the diffusible compound diffuses into the layer (II) and is almost uniformly re-distributed in the layer (I) and layer (II). Second, the diffusible compound thus diffused into the layer (II) can be removed by dissolving off with a hydrolysis solution by applying thereto a hydrolysis treatment, but even when such a treatment is applied, the diffusible compound diffuses into the layer (II) from the layer (I) during the period prior to the image-receiving material being practically used by users after production thereof. This adversely affects the photographic characteristic, and if the amount of the diffusible compound contained in the layer (I) is reduced in order to reduce these disadvantages, a sufficient modification of photographic properties of the transferred images cannot be obtained. The above-mentioned difficult problems occur because the layer (I) and layer (II) are composed of very similar components, such that an interface is not formed between the layers, whereby materials in each layer easily diffuse into both layers.